An inkjet printing system typically includes one or more printheads and their corresponding ink supplies. A printhead includes an ink inlet that is connected to its ink supply and an array of drop ejectors, each ejector including an ink pressurization chamber, an ejecting actuator and a nozzle through which droplets of ink are ejected. The ejecting actuator may be one of various types, including a heater that vaporizes some of the ink in the chamber in order to propel a droplet out of the nozzle, or a piezoelectric device that changes the wall geometry of the ink pressurization chamber in order to generate a pressure wave that ejects a droplet. The droplets are typically directed toward paper or other print medium (sometimes generically referred to as recording medium or paper herein) in order to produce an image according to image data that is converted into electronic firing pulses for the drop ejectors as the print medium is moved relative to the printhead.
Motion of the print medium relative to the printhead can consist of keeping the printhead stationary and advancing the print medium past the printhead while the drops are ejected. This architecture is appropriate if the nozzle array on the printhead can address the entire region of interest across the width of the print medium. Such printheads are sometimes called pagewidth printheads. A second type of printer architecture is the carriage printer, where the printhead nozzle array is somewhat smaller than the extent of the region of interest for printing on the print medium and the printhead is mounted on a carriage. In a carriage printer, the print medium is advanced a given distance along a print medium advance direction and then stopped. While the print medium is stopped, the printhead carriage is moved in a carriage scan direction that is substantially perpendicular to the print medium advance direction as the drops are ejected from the nozzles. After the carriage has printed a swath of the image while traversing the print medium, the print medium is advanced, the carriage direction of motion is reversed, and the image is formed swath by swath.
Inkjet ink includes a variety of volatile and nonvolatile components including pigments or dyes, humectants, image durability enhancers, and carriers or solvents. Inkjet inks used in printers for the home or office typically include a high percentage of water—on the order of 80%. Water can interact with the paper being printed on to cause the paper to curl, due to differential stresses on the printed surface and the non-printed surface for pages printed with relatively high ink coverage on one side of the paper. Curl can appear immediately after printing or it may take a day or so to appear. In a severe case of curl, the paper sheet can roll up like a scroll so that it cannot be stacked sheet upon sheet. In addition to the amount of ink coverage, another important factor affecting the severity of curl is the type of paper. Many types of papers designed for inkjet printing are made to have small built-in differential stress between printed and unprinted sides after printing and show little curl even for high ink coverage. Such papers are typically thicker and have higher mechanical strength than so-called plain papers that are for general use and not optimized for inkjet printing. However, some of the specially designed papers for inkjet are significantly more expensive than plain papers, so that the user may choose to use plain papers for many print jobs. While plain paper can be satisfactory for low amounts of ink coverage, for example typical text printing, there can be an objectionable amount of paper curl when printing color graphics or photographs.
A variety of approaches have been used to reduce the amount of curl. In some piezoelectric inkjet printers an anti-curl solution is added to the inks. However this typically causes the inks to be somewhat viscous. Such a solution is typically not feasible for thermal inkjet printers. U.S. Pat. No. 7,208,032 and U.S. Pat. No. 7,604,344 disclose an inkjet printing apparatus having a coating roller to apply an anti-curl solution to the paper after it is picked from the paper input tray and before it reaches the printing region. However, such an architecture can be complex and costly and in some instances can apply anti-curl solution whether it is needed or not, so that it can be wasteful and require objectionably frequent replacement of anti-curl solution by the user. U.S. Pat. No. 5,633,662 discloses selecting a maximum ink volume per pixel to provide good color coverage while avoiding paper curl, bleeding, etc. While this method avoids the use of anti-curl solution, it is inherently limited in the intensity of printed images that can be produced. U.S. Pat. No. 5,764,263 discloses printing an optically clear aqueous liquid containing anti-curl agents on the opposite side of the paper from a printed image. While this can be effective, it results in an overly complex and bulky printing system.
What is needed is a simple low-cost printing system and method of printing that can be used to reduce curl to acceptable levels in low-cost inkjet carriage printers without compromising print quality, and without applying anti-curl solution in a wasteful manner.